A new class of high molecular weight organosilicon polymers which have excellent physical, thermal and electrical properties and outstanding resistance to water, and that can be used to prepare shaped articles is described in parent applications Ser. Nos. 07/079,740 and 06/901,092. They are thermoset or thermoplastic organosilicon polymers comprising alternating polycyclic hydrocarbon residues and cyclic polysiloxanes or tetrahedral siloxysilane residues linked through carbon to silicon bonds. This application is directed to novel organosilicon prepolymer compositions that can be used to prepare such polymers, blends of such prepolymers with .tbd.SiH bearing polysiloxanes and siloxysilanes useful for making thermoset organosilicon polymers, the processes of making the prepolymers and the blends, and the processes of preparing thermoset organosilicon polymers from such blends.